Membrane separators used in membrane separation activated sludge process for biologically treating waste water, drainage water etc have a plurality of membrane cartridges. As shown in FIG. 11, a membrane cartridge 51 has filtration membranes 53 disposed on the front and rear surfaces of a filter supporting plate 52 made of resin, and the entire peripheral portions of the filtration membranes 53 are fused to the filter supporting plate 52 by heating with a heater or by ultrasonic vibration. A transmitted liquor passage is present between the filter supporting plate 52 and each filtration membrane 53. A collecting hole 54 communicated to the transmitted liquor passage extends from front to rear of the filter supporting plate 52. A transmitted liquor outlet 55 communicated to the collecting hole 54 is disposed at an upper end of the filter supporting plate 52.
In the membrane separator, the plural membrane cartridges 51 are oriented vertically and in parallel with one another to form a passage between the adjacent membrane cartridges 51. Each membrane cartridge 51 is connected to a passage for discharging the transmitted liquor via a tube connected to the transmitted liquor outlet 55. In the membrane separator, the filtration membranes 53 perform filtration by using a differential pressure between membranes that is exerted between the front and rear of each filtration membrane 53. The membrane separator can be operated by forced suction system of applying driving pressure as the suction pressure exerted by a suction pump, or gravity filtration system of applying driving pressure as the water head in the tank.
The membrane separator has an air diffuser disposed under the membrane cartridges 51. For example, the air diffuser is obtained only by boring a plurality of holes of approximately φ10 mm in a lower part of a pipe made of vinyl chloride.
In membrane separation activated sludge process, the membrane separator is immersed in an aeration tank, and an intra-tank mixed liquor containing the activated sludge in the aeration tank is aerated by the air diffused from the air diffuser. The diffused air generates an upstream of gas-liquid-solid mixed phase by air lift action. The upstream supplies the intra-tank mixed liquor to the filtration membranes 53 of the membrane cartridges 51 of the membrane separator, and the filtration membranes 53 performs crossflow filtration of the intra-tank mixed liquor. The crossflow of the upstream of the intra-tank nixed liquor on the membrane surfaces enables to suppress cake from attaching to the membrane surfaces of the filtration membranes 53 and flush the membrane surfaces.
Flushing of the membrane surfaces in the membrane separator requires air supply of averaging not less than 10 L/min for every membrane cartridge 51. Unless the air is supplied uniformly to the entire membrane surfaces of the membrane cartridges 51, the local area of the filtration membranes 53 receiving much air is subjected to a large load, which lowers the lifetime of the membranes. On the other hand, the local area of the filtration membranes 53 receiving less air fails to obtain a sufficient flushing effect, which accelerates the attachment of cake to the membrane surfaces.
Supplying a predetermined amount of air from the air diffuser ensures oxygen necessary for biological treatment in membrane separation activated sludge process. The predetermined amount of air is set based on the efficiency of oxygen dissolution into activated sludge. Increasing the efficiency of oxygen dissolution is desirable for achieving power saving that is realized by downsizing a blower supplying air to the air diffuser and reducing its driving time.
Therefore, it can be considered that a uniform dispersion of the diffused air and improvement in the efficiency of oxygen dissolution are achieved by arranging blowholes having a small diameter at a plurality of locations of the air diffuser. With this air diffuser, it is however difficult to completely avoid that the blowholes and a diffuser tube become clogged by activated sludge. When the air diffuser becomes clogged, it is difficult to perform biological treatment with activated sludge. It follows that the filtration membranes 53 of the membrane cartridges 51 in the membrane separator become clogged by the cake attached to the membrane surfaces. This impairs the function of the membrane separator.
It is therefore necessary to flush the air diffuser in a periodic maintenance as measure to prevent clogging of the air diffuser. This flushing is usually performed about one time for two weeks. Letting water in the diffuser tube of the air diffuser flushes the diffuser tube. To perform such flushing, it is however necessary to place ancillary facility such as a flushing water tank and a flushing pump.
In the membrane separator, the operation of the suction pump is stopped when the waste water flow is reduced and the amount of waste water in the tank is below a predetermined water level. In gravity filtration type that uses the water head in the tank as a driving pressure, the operation stops spontaneously when the amount of flow is reduced and the water head is lower than a predetermined value.
The amount of waste water flowing to the waste water treatment facility is not always constant. That is, the facility receiving a large daily variation or seasonal variation may have little or no daily amount of waste water flow in some cases and have an excessively large daily amount in some cases. Therefore, with respect to the aeration tank of a general waste water treatment facility, the ratings of the tank capacity and the air diffuser are set in such a redundant specification as to comply with an abnormally large waste water flow.
This contributes to a considerable variation of water level in the aeration tank between the time of a maximum load and the time of a minimum load of waste water flow. At the time of a low load, the operation of the membrane separator is brought to a stop. When the aeration of activated sludge with the air from the air diffuser is stopped for a long period of time, there may occur oxygen deficiency and insufficient stirring of the intra-tank mixed liquor. It is therefore necessary that only air diffusion operation for diffusing air from the air diffuser be performed in the state of stopping filtration operation of the membrane separator. However, this operation state causes damage to the filtration membranes 53 of the membrane cartridges 51.
On the other hand, during the period of time that the operation of the air diffuser is stopped, sludge flows to the diffuser tube through the blowholes. This sludge is dried with air during air diffusion operation, and the dried sludge causes clogging of the blowholes.
Accordingly, the present invention has for its object the provision of an air diffuser and a method of flushing an air diffuser that can structurally suppress a clogging of a diffuser tube and flush the diffuser tube by a simple manipulation.